Low thermal expansion glasses



Oct. 1, 1968 w. H. DUMBAUGH, JR 3,404,015

LOW THERMAL EXPANSION GLASSES Filed April 28, 1965 2 Sheets-Sheet 1 LINEAR EXPANSION 0F GLASSES WITH VARYING TEMPERATURE SODA LIME 3000 AEFERRED GLASS SILICA LINEAR EXPANSION-AL/L PARTS PER MILLION TEMPERATURE C FIG. 1

INVEN TOR. William H. Dumbaugh, Jn

BY Mam ATTORNEY VISCOSITY-POISES Oct. 1, 1968 Filed April 28, 1965 w. H. DUMBAUGH, JR 3,404,015

LOW THERMAL EXPANSION GLASSES 2 Sheets-Sheet 2 VISCOSITY OF GLASSES IN THE MELTING TEMPERATURE RANGE PREFERRED GLASS a v IO 2 e00 900 I000 I100 I200 I300 |400 I500 TEMPERATURE c FIG. 2

INVENTOR.

William H. Dumbaugh, Jr

BY WKW ATTORNEY Patented Oct. 1, 1968 3,404,015 LOW THERMAL EXPANSION GLASSES William H. Dumbaugh, Jr., Corning, N.Y., assignor to Corning Glass Works, Corning, N.Y., a corporation of New York Filed Apr. 28, 1965, Ser. No. 451,517 7 Claims. (Cl. 106-52) This invention relates to low thermal expansion glass compositions based on a magnesium oxide-aluminum oxide-silica system. In one specific aspect it relates to a composition for making low thermal expansion telescope mirror disks.

One of the great achievements in the use of glass has been in the making of the glass mirror disk for the 200 inch reflecting telescope at Mount Palomar. In designing an astronomical mirror it is important to determine what effect temperature changes will have on it. The surface of the mirror is so finely focused to the light of the stars that even the slightest distortion caused by expansion or contraction of the mirror body may render the mirror temporarily useless. It is known that thick pieces of glass do not respond evenly to heating and cooling. As the glass begins to cool from the heat of day, complicated strains are produced in the glass which distort the face of the mirror from its true curvature such that proper focusing of images on the telescope may then be impossible.

Heretofore, low expansion glasses have been successfully used in the manufacture of telescope mirrors. These glasses are very nearly insensitive to temperature changes. Well-known low thermal expansion glasses are certain borosilicate glasses sold commercially by the Corning Glass Works under the trademark Pyrex brand glass. Some of these, such as for example, glass Code 7740, have been widely and successfully used for laboratory ware and cooking utensils subjected to rapid changes in temperature. Another borosilicate glass composition similar to Pyrex brand glass is sold commercially under the trademark Duran 50.

The glass composition used in making the disk for the Mount Palomar telescope was a special low expansion glass described in the Hood patent U.S. 2,106,526. This glass, designated as glass Code 7160, has a very low thermal coefficient of expansion of 24.5 X 10- per degree C., a high chemical stability and sufficient surface hardness. While this glass is of a special composition designed for a special purpose, the process of making large disks from the composition is extremely complicated and it is impossible to make such disks by ordinary means. One great difficulty involves the fact that the glass composition has an extremely high viscosity.

Unfortunately, making borosilicate glasses of high optical quality is also extremely difiicult. Numerous cords and striae are found in regular melts of borosilicate glasses and these generally do not meet optical standards. In addition the borosilicate glasses tend to phase separate and require a carefully controlled annealing schedule. It would be desirable to improve the glass quality by melting the glass composition in a continuous glass furnace used to make optical glass, such as described by C. J. Phillips in Glass Its Industrial Applications, 182, Reinhold, New York, 1960. This furnace contains a platinum lined finer to minimize stones, bubbles and striae. However, at a melting temperature of 1500- 1550 C. the viscosity of the borosilicate glasses generally is too high to obtain homogeneous compositions.

Quite surprisingly, I have discovered a novel glass composition Which has the excellent properties of the borosilicate glasses and in addition can be melted in a continuous glass furnace for melting optical glass. The composition is based on a magnesium oxide-aluminum oxidesilica system. Glasses made according to the present invention have a low thermal expansion coefficient, are capable of being melted in an optical unit, have no phase separation, and have good acid durability.

It is therefore an object of the present invention to provide a glass composition capable of being molded into a low thermal expansion telescope mirror disk.

It is a further object of the present invention to provide a low expansion glass composition capable of being melted in a standard continuous glass furnace for making optical glass.

In accordance with the present invention I have discovered a low thermal expansion glass composition consisting essentially on the oxide basis of magnesium oxide 6-12 mole percent, aluminum oxide 8-18 mole percent, silica 64-70 mole percent, alkali metal oxide 0.5-3 mole percent and a member selected from a group consisting of antimony trioxide and tantalum oxide 1-3 mole percent. The composition may additionally also contain boric oxide 0-5 mole percent, and Zinc oxide 0-5 mole percent.

Silica, in addition to its customary role as a network former, when present in large amounts imparts to the glass a low thermal expansion. It is necessary that the amount of silica be high in the range of 64-70 mole percent. When the amount of silica falls below 64 percent the thermal expansion of the glass becomes too high for the intended use and the chemical durability decreases. While it is desirable that the silica content be as high as possible, above 70 percent the viscosity of the glass composition becomes too high for melting in an optical glassmaking furnace to produce a glass of high quality.

The amount of alumina present in the composition is closely related to the silica content. Generally, the total amount of alumina and silica present should be in the range of -85 mole percent. Considered separately, the amount of alumina should be in the range of 8-18 mole percent. When less than 8 percent of alumina is present, the amount of silica would have to be increased resulting in a glass which is very hard to melt; on the other hand, the liquidus of the glass becomes too high and the glass becomes unstable when the alumina content is greater than 18 mole percent. Small amounts of boric oxide from 0-5 mole percent may be present in the composition as a flux, however, large amounts of boric oxides must be avoided since it decreases the slope of the viscosity temperature curve and tends to produce a phase separation upon heating.

The primary modifier used in the composition of the present invention is magnesium oxide. An amount of 6-16 mole percent of magnesium oxide should be present in the glass composition. The viscosity of the composition becomes too high for melting good quality glass when the amount of magnesium oxide falls below 6 mole percent, whereas above 16 mole percent of magnesium oxide undesirably increases the thermal expansion. Other alkaline earth metal oxides cannot be substituted on a mole for mole basis for magnesium oxide since these tend to raise the expansion coeflicient. Another modifier, which can be added in small amounts, is zinc oxide which may be present in an amount of 0-5 mole percent. However, large amounts of zinc oxide should be avoided since it tends to undesirably raise the viscosity of the melt.

A very limited amount of alkali metal oxides, preferably lithium oxide, should be used. It is necessary that 0.5 mole percent be present in the composition to inhibit phase separation and to help improve meltability by lowering the viscosity. More than 3 percent should be avoided since the thermal expansion becomes too high.

The presence of antimony trioxide and/or tantalum oxide is very important in the composition of the present invention. These should be present in amounts ranging from about l3 mole percent, corresponding in weight to about 412 percent, to achieve the proper combination peratures of the preferred glass is compared to low expansion Pyrex glasses.

Expansion coefiicient of expansion, viscosity, chemical durability, and thermal Code: (300 C.), per C. stability. At least one mole percent must be present, but 5 7160 24.5)( over three mole percent raises the thermal expansion un- 7740 32x10 desirably. While very small amounts of antimony tri- 7250 36 10- oxlfie have used as .finmg Such a The curves of these borosilicate glasses indicate that at antlmony mox} e comm? 1S uncfnninon commercial the same temperatures as the viscosities of the composiglasses Replacmg g i tnoxlde. pamanypr 10 tions decrease, the expansion coefiicients correspondingly pl.etely with arsemc tnoxldfi and arsemc pemmflde were increase. Only Code 7250 of the borosilicate glasses tried. Unfortunately, these melts frothed excessively and Shown has a Viscosity low enough in the melting range a scum was formed on the surface of the melt. The quallty of about to be capable of being melted in an of the arsemc substltltebd dglass was also poor due to the optical furnace. The curve of the preferred glass, however, presence of cords an has a much greater slope than the borosilicate glasses,

For Purposes of the present. f the thermal and intersects both the curves of Code 7740 glass and expansion is defined as the coeflicient of lmear expansion Code 7250 glasa In the melting temperature range the IS the g j g m i fi f preferred glass is even less viscous than Code 7250 glass t ength at T 8 Va 0 t e we Clem Wines indicating that the preferred glass can be melted in an With temperature and may be illustrated by the equation: Optical furnace 1:1 I at My invention is further illustrated by the following t 0 wherein it is the temperature in degrees C., I is the length examp EXAMPLE I at temperature 1, I is the length at 0 C. and a is the coefiicient of linear expansion. The coeflicient of linear The preferred glass composition of the present invenexpansion of the glasses prepared according to the present tion was prepared and melted from the following invention should be no greater than about 10- per formulation: degree C. over the range O300 C. TABLE HI The preferred composition of the applicants invention i b h d Welght g fi is set forth in the table below. The ingredients are given 30 i h San 17 4 in terms of the corresponding oxides. 25 2; rate TABLE I.PREFERRED COMPOSITION 225: Oxide Mole Percent Weight Percent Zi xid 18 96 3 59,38 Martinsburg petalite 76.32

13g Antimony trioxide 33.92

2 1 2 2 The melting was performed in a platinum crucible at 2 0.87 a temperature of 1550 C. for four hours. The glass was 2 40 then cast into disks having a diameter of eight inches and a thickness of four inches. Under these conditions the lass was clear, ver sli htl seed and containe o e The accompanying drawing illustrates the improvement :Ords' The melt g i s s to m h glass q g prepared accordmg to the pres about four hours and then to below the strain point at ent invention in W ic 10 per hour. The disk was annealed b heatm to a FiGURE 1 shqws a of the thermal temperature of 680 C. at 8 C. per 1101 1 1 soaki ng at efiicient of expansion at various temperatures of the pre- 00 for 20 hours Cooling at 30 C per g to C ferred glass prepared according to the present invention and Cooling to room tempefamre at 80 C. Per houn 8 and crtam Pnor art glasses evidence of phase separation was observed in the mass of FIGURE 2 shows a comparison of the viscosity at var1- the glass after the Cooling schedu1e ous temperatures of the preferred glass comp and The glass composition was then tested using standard certam Pnor at glass composltlons; techniques to determine its characteristics. The properties The composltlons of 10W ,expanslon Pnor glasses of the preferred glass are set forth in the table below and used for purposfi's of comPanson are shown In table are compared to the commercial low expansion glasses below and are given 1n weight percent on the oxide bas1s. discussed above.

TABLE IV.PROPERTIES OF TELE TABLE II.COMPOSITION SCOPE DISK GLASSES Pre- Code Duran Ingredient 7160 7740 Duran 50 ferred 7160 50 SiOz 81.0 80.27 79.69 Expansion Coefficient (25-300 o. 29 25 3M B203. 16. 5 12. 23 10. 29 10- o AlgO3 0. 9 2. 79 3. 10 Working Point, C-.. 1, 235 1,385 1, 245 NazO 1. 3 3. 97 5. 20 Softening Point, 0.- 850 330 15 K20 0. 40 Annealing Point, C. Li 0.3 Strain Point, C Ca0. 0.80 0.77 Density, g./cm. MgO.. 0.87 Liquidus, C

Referring to FIGURE 1, the thermal expansion of the low expansion glasses lies between an extremely low expansion 96 percent silica glass (Vycor glass) and a high expansion soda lime glass (Code 0080 glass). It can readily be observed that the preferred glass composition shows improvement in expansion characteristics over Code 7740 glass composition.

Referring to FIGURE 2, the viscosities at various tem- Youngs Modulus, l0- p.s.i Poisson's Ratio Refractive Index:

Birefringence Constant, mu/emJkgJmmP- *Approximate.

Further tests were performed to determine the chemical durability of the glass composition. These tests indicate to melt. The glass compositions are set forth in the table I below wherein the percents are given on the oxide basis:

GLASS COMPO SIIIONS Ex. II Ex. III Ex. IV Ex. V Ex. VI Ex. VII

Mole Weight; Mole Weight Mole Weight Mole Weight Mole Weight Mole Weight percent percent percent percent percent percent percent percent percent percent percent percent Expansion coefficient 300 C.) 10 C 29. 7 28. 5 28. 4 28. 0 30. 0 39 Annealing Point, C. 724 710 700 081 681 071 Strain Point. 0...- 070 034 951 029 035 023 Liquidus, C 1,405 1, 355 1, 315 1,328 1, 324 1, 329 Plate Durability, 5% H01,

24 hrs., 95 0., wt. loss (m /ems 4. 82 1. 19 0. 94 0. 09 0. 79 0. 04

the property of resistance to chemical reagents. The table I claim: below shows the results obtained. 1. A low thermal expansion glass composition, having 05 a maximum coefficient of thermal expansion of about TABLE V CHEMICAL DURABILITY OF DISK GLASSES 10" per degree C., cons1st1ng essentially on the oxlde A. Polished Plate Test basis of magnesium oxlde 6-12 mole percent, alumlnum Glass Reagent Temp Time Wt Loss oxide 8-1 8 mole percent, silica 64-70 mole percent, alkali *0. hrs. mtL/em. metal ox1de 0.53 mole percent and antimony trioxlde Preferred 5% H01 soln 95 24 0.68 30 Oxide molfi p ffl l Sode 71 %0 2Z0 gg lsopi- 8g 1.8% 2. The composition of claim 1, containing boric oxide uran a ll. 3 Preferred 5Z2 IIga8 95 6 1.36 T1316 Percent f l 1 d Code 7100 5. a 95 0 2. e com osition 0 c aim co'1tai i zin oxi Duran 50 5 NaOH 95 6 5.36 0 5 1 l n ng c e Preferred... 50 2 2) 95 a 0. 4 51 Percent f i h h Code 7100... 150 4.. 05 com osition a m w re' t u Duran 50 ODZNNQQCOZ 95 0 2.49 f e p 0 mt e Otalamo m 0 811108. and aluminum oxide are In the range of -85 B. Powder Tests mole percent T P 5. The composltlon of claim 2, containmg ZlllC oxide ,ff fgfl 0-5 mole percent. Glass Reagent Te mp Tlime, lgacgled 40 6. The composition Of claim 1, wherein said alkali C fiz metal oxide is lithium oxide.

N920) 7. A low thermal expansion glass composition consist- Prefmed OWN Hismnfluu 4 (10.23 mg essentially on the oxide basis of magnesium oxide 8 Duran 50 0.02 N H2804 90 4 0. 30 mole percent, aluminum oxide 12 mole percent, silica 68 Preferred HzO 90 4 0. 0005 1 b Dm-mso H20 90 4 M59 45 me e percent, 0r1c 0x1 e 4 mole percent, Zll'lC OXlde 4 mole percent, llthium oxide 2 mole percent, and antimony trioxide 2 mole percent.

AMPLES II- I EX V I References Cited 1Following the proicedureh orfl llilxample I, ai series of 0 UNITED STATES PATENTS g asses were prepare In W 1c t e amount 0 antimony 3 275 493 9/1966 M cDoWell 106.39 X oxide was increased from zero to thre mole ercent 1th e p W 3,282,711 11/1955 Lin -39 a corresponding decrease in the amount of aluminum oxide. The results indicated that in the absence of antimony oxide the glass product has poor durability, has ahigh liquidus, tends to phase separate, and is more difficult HELEN M. MCCARTHY, Primary Examiner.

W. R. SATTERFIELD, Assistant Examiner. 

1. A LOW THERMAL EXPANSION GLASS COMPOSITION, HAVING A MAXIMUM COEFFICIENT OF THERMAL EXPANSION OF ABOUT 30X10-7 PER DEGREE C., CONSISTING ESSENTIALLY ON THE OXIDE BASIS OF MAGNESIUM OXIDE 6-12 MOLE PERCENT, ALUMINUM OXIDE 8-18 MOLE PERCENT, SILICA 64-70 MOLE PERCENT, ALKALI METAL OXIDE 0.5-3 MOLE PERCENT AND ANTIMONY TRIOXIDE OXIDE 1-3 MOLE PERCENT. 